In conventional technologies, birefringent polymer materials have been used for optical compensation or the like of liquid crystal displays. Such optical compensation materials that are widely used include plastic films that have undergone stretching or the like so that they have birefringence. In recent years, an optical compensation material including a substrate coated with a polymer having high birefringence-producing capability, such as aromatic polyimide or aromatic polyester, has also been developed (see for example Patent Documents 1 and 2).
Such an aromatic polymer is characterized by having a high level of heat resistance and mechanical strength but tends to have low solubility in organic solvents. Therefore, an optical film mainly composed of such an aromatic polymer is generally formed by a process including the steps of dissolving the polymer in a high-polarity solvent, which therefore has high solubility, to form a solution, and then applying the solution to a metallic drum or metallic belt or a base film or the like and drying it to form a film. In such a film production method, however, since a choice of solvents capable of dissolving the polymer is limited, drying conditions may be restricted, or expensive equipment may be needed. Since the substrate used in the coating process has to be insoluble in the solvent, materials usable for the substrate are also limited. From these points of view, it has been demanded to develop a polymer that is soluble in a low-polarity solvent such as toluene or ethyl acetate and has birefringence-producing capability so that it can function as an optical compensation material.
Patent Document 1: the pamphlet of PCT International Publication No. WO94/24191
Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. 2004-070329